During the printing process, a selected image is repeatedly printed onto the surface of a substrate. A substrate is often provided in web form, a continuous roll of a material, or sheet form, separate sheets of substrate. The substrate is capable of receiving print while traveling through a printing press. To ensure the quality and accuracy of the printed image, various methods and devices have been developed for monitoring the color and location of the ink that has been applied to the substrate. One such method includes placing a color bar, i.e. small quantity of individual ink components, on the substrate such that a color sensing camera can be positioned over the color bar. The camera can then scan the color quality and location accuracy of the individual ink patches represented in the color bar. If any color or location inaccuracies are detected in the color bar, a proportional inaccuracy may be present in the remainder of the printed image. Another method includes locating a color location without dedicated color marks or patches and instead using portions or regions of interest of the work being printed.
However, the current methods and devices for monitoring color accuracy during application of inks to a substrate are less accurate when the substrate is transparent and/or translucent, e.g. where the substrate is a thin plastic sheet. Color measurement is dependent on whether the substrate is transparent, translucent, or opaque. Transparent materials allow all or substantially all of the light rays to pass. Characteristic of this property is that transparent materials can be seen through; that is, they allow images to pass. The opposite property is opacity. Opaque materials do not allow any light rays to pass through because all of the light rays are being reflected or absorbed. Thus, objects cannot be seen through opaque materials. Translucent materials, on the other hand, allow some light rays to pass through the material while others are either absorbed or reflected. Because translucent materials allow light to pass through them diffusely, the image when seen through a translucent material is generally distorted. Transparent and translucent substrates are otherwise referred to herein as transitive substrates. The degree of transparency or translucency is measured by percent opacity, where a material is opaque at 100% opacity and transparent at 0% opacity. As opposed to printing on a paper substrate, a transitive substrate provides no discernable or consistent color background and therefore results in inaccurate color monitoring. Shadows are also created as a result of light being able to pass through the substrate. Previous attempts to remedy this shortcoming have included placing a background under the substrate. The background has been a ceramic tile having a predetermined color, usually white or black. The background may be positioned below the moving substrate to provide the necessary field for accurately monitoring the colored ink in the color bar. However, if the substrate is elevated above the ceramic tile during the scanning process, shadowing may result that will interfere with the accuracy of the image data received by the color sensing camera. Specifically, light used to illuminate the image for the camera may pass through the substrate at an angle creating shadows under other areas of the image and/or color bar. Accordingly, it is desirable to minimize the spacing between the substrate and the background target.
However, it is often advisable to maintain some spacing during normal operation. Due to the abrasive nature of the moving substrate and in an effort to reduce the potential for ink transfer to the ceramic tile, contact between the substrate and the ceramic tile is desirably minimized.
Further inhibiting the application of current methods and devices for monitoring the application of colored inks to substrates, such as a transitive web, is the lack of surface stability within the transitive web. As the substrate is under tension during the printing process, small ripples or variations in the surface of the substrate may easily occur. Any inconsistency in the surface, height or tension of the substrate located in view of the color sensing camera may alter the accuracy of the image data received by the color sensing camera. Accordingly, it is desirable to avoid unnecessary application of force to the transitive substrate while simultaneously providing sufficient stability to the portion of the transitive substrate being scanned by the color sensing camera.